This is quite simple. There are two types of SSD drives. They are EIDE and SATA. If you find an EIDE SSD drive, and you can find plenty on Amazon's site, plop it in the computer, and it will add some speed to your PC. You just gotta put the right SSD drive with the right form factor. Install an EIDE SSD inside an older model laptop or desktop, which would take EIDE drives, and install a SATA SSD inside a newer model laptop or desktop, which takes SATA drives. Make sure you order a 2.5" drive for your laptop, or a 3.5" drive for your desktop, unless your desktop has a bracket that can fit the 2.5" drive. The SATA SSD drives are less expensive than the EIDE SSD drives, since all EIDE drives are being phased out. The size of the ALL SATA drives are bigger compared to EIDE drives. Also, the size of desktop drives aare bigger than laptop drives, no matter if it's a SATA or EIDE. SATA drives are faster than EIDE ones, but EIDE SSD drives are faster than conventional EIDE drives, just as SATA SSD drives are faster than conventional SATA drives. Of course, when I say conventional, I mean drives with moving parts. SSD drives are only drives with flash memory inside them. They last a lot longer than conventional drives.
Hope this helps!